There have been proposed many plain display devices which employ a liquid crystal, plasma or electroluminescent thin film. Among them, the thin film electroluminescent devices are mainly used for displays of office automation apparatus or testing apparatus, because the device is light and thin. Recently, the devices which have a display capacity of 1,152.times.900 dots and a gradation function of 16 steps have been commercially available. Thus, as increasing informations to be displayed, the thin film electroluminescent devices having larger area and larger size are desired.
The electroluminescent devices which have been practically used have an electroluminescent layer of ZnS:Mn which is generally produced by an electron beam deposition method. However, for producing the devices having such larger area and larger size, the electron beam evaporation method encounters some problems. For example, a gas exhausting system is employed in the method, but it is required that its exhausting capacity be increased with the increase of the size of the substrate. It is however difficult to make such exhausting system having a large exhausting capacity. It is also reported in Applied Physics 51, 821 (1982) that a temperature of the substrate changes the Mn concentration or the quality of the ZnS:Mn film prepared by the electron beam deposition method. However, it is very difficult to keep the temperature of the substrate throughout the large surface area.
A chemical vapor deposition (CVD) method is also known to the art as a method forming a uniform thin film on a large area and is popular in the semiconductor industry. FIG. 5 (a) schematically shows an apparatus for the CVD method and FIG. 5 (b) schematically shows a substrate holder. In the apparatus, a source material gas is introduced from introducing tubes 1, 2 and 3 which are arranged at one end of a reaction tube 4. The reaction tube 4 is exhausted by an exhausting system through the other end to keep the reaction tube 4 in vacuo. The reaction tube 4 is surrounded with furnaces 5 which control a temperature of substrates 8. The substrates 8 are held by a holder 7 in a uniform by heating area 6 in which a certain temperature is kept fixed. If the source material gas is composed of SiH.sub.4 and NH.sub.3, an Si.sub.3 N.sub.4 film which is employed as an insulation film for LSI is formed by the following reaction: EQU SiH.sub.4 +4/3NH.sub.3 .fwdarw.1/3Si.sub.3 N.sub.4 +4H.sub.2
In the case where the CVD apparatus is operated in an industrial scale, many substrates are put in the uniformly heating area 6, but the non-uniformity of the deposition film thickness generally occurs because of two reasons. One reason is the diffusion constant of the source material gas and the other reason is the concentration difference of the source material gas in the flowing direction in which the source material is consumed more at earlier contacting place with the substrate. The former reason results in the non-uniformity of the deposited film thickness throughout the surface and the later reason results in the reduction of film thickness along the source gas stream. The former problem, however, can be removed by reducing an inside pressure of the reaction tube and increasing the diffusion constant, and the later can be removed by increasing a feeding rate of the source material in terms of the source material gas flow in comparison with the consuming rate of the source material so as to almost unify the source material gas concentration in the flowing direction.
The CVD method has been applied little for forming a luminescent layer of the thin film electroluminescent device, but the present inventors tried to study it for forming the luminescent layer larger around the thin film electroluminescent devices and for producing them in an industrial scale. The CVD apparatus as shown in FIG. 5 was employed for forming a ZnS film. ZnS powder was heated at 900.degree. to 1,000.degree. C. and the vapor was sent to the reaction tube 4 with H.sub.2 gas. The glass substrate was employed and its temperature was set 450.degree. to 550.degree. C. An inside pressure of the reaction tube 4 was kept 10.sup.-2 torr and the deposition was conducted for 60 minutes. The obtained ZnS film was a polycrystalline film having zincblende structure and oriented to the (111) direction. FIG. 6 (a) is a film thickness distribution in the uniformly heating area, which clearly shows the uniformity of the film thickness.
Next, a ZnS:Mn film was prepared using the same apparatus. The deposition conditions were the same, provided that an Mn gas source was fed. The Mn gas was introduced in the form of MnCl.sub.2 gas prepared by heating Mn at 800.degree. to 900.degree. C. and then mixing with HCl gas to react as follow; EQU Mn.sub.(S) +2HCl.sub.(g) .fwdarw.MnCl.sub.2(g) +H.sub.2(g)
FIG. 6 (b) is a graph which indicates an Mn concentration and a ZnS:Mn film thickness distribution in the uniformly heating area. The results shows that the film thickness reduced to 1/3, but has good uniformity. The Mn concentration is widely changed from higher at the upper stream to lower at lower stream. The ZnS:Mn film has a wultzite structure. It is believed that the reduction of the film thickness is caused by etching the ZnS film by means of HCl gas which occurs by the addition of MnCl.sub.2 gas.
As is apparent from the above results, the ZnS:Mn film obtained by the reduced-pressure CVD method has a drawback of the non-uniformity of the Mn concentration. The above results are obtained by a horizontal furnace, but if it is conducted by a perpendicular furnace, the same non-uniformity of Mn concentration would occur within the substrate.
It is believed that the non-uniformity of Mn concentration is caused by the following reason: Since a vapor pressure of MnCl.sub.2 is low in the substrate temperature area, the deposition of Mn occurs easily, so that the Mn in the source material gas is consumed more at the upper stream and the Mn concentration reduces at the lower stream. The non-uniformity may be improved by increasing an flow rate of the source material gas, but the source material gas is sufficiently fed in view of the uniformity of the ZnS:Mn film thickness. If the flow rate of the source material gas is increased, a conversion rate of the source material to the deposited film is also reduced and a production cost becomes expensive. Also, the increase of the flow raises an inside pressure of the reaction tube, which often gives rise to an non-uniformity caused by the increase of diffusion constant and adversely affects on the uniformity of the ZnS film which is a host material of the ZnS:Mn film.
The above mentioned problems are common with the luminescent center of the thin film electroluminescent film device produced by the reduced-pressure CVD method, i.e. Tb in ZnS:Tb, Eu in Cas:Eu film and Ce in SrS:Ce.